The present invention relates to an exhaust gas purification system which purifies harmful substances contained in exhaust gas of an engine, and more particularly to a catalyst deterioration detecting apparatus for accurately detecting a deteriorated condition of a catalyst used for oxidizing combustible substances (e.g., HC) contained in the exhaust gas of a diesel engine. The present invention also relates to a catalyst deterioration alarming apparatus for alarming the deterioration of the catalyst.
The gasoline engine is generally equipped with an oxygen concentration sensor to detect the oxygen concentration in the exhaust gas and to control an air-fuel ratio of gas mixture introduced into a combustion chamber of this engine. It is conventionally known that the oxygen concentration sensor can be used to detect deterioration of a ternary catalyst.
Diesel engines are characterized in that the air-fuel ratio is set to a lean region compared with a theoretical air-fuel ratio. The catalyst deterioration judging method employed for gasoline engines cannot be directly applied to the diesel engines.
It may be useful to use a hydrocarbon (HC) sensor to judge the deterioration of a catalyst. However, a reliable HC sensor has not been developed yet. And also, the HC sensor will be expensive.
Japanese patent No. 2593506 discloses a conventional method for judging the deteriorated condition of a catalyst based on a difference between an upstream exhaust gas temperature and a downstream exhaust gas temperature of a catalytic converter. This judging method is non-expensive because the exhaust gas temperature sensor equipped in a catalyst container can be utilized for this judgement. However, this judging method has not been practically used because the detection accuracy is not yet assured for every operating mode of an engine and also because the measuring accuracy of each exhaust gas temperature sensor is not yet reliable.
Unexamined Japanese patent publication No. 3-50315 discloses a technique for optimizing the detecting point for improving the detecting accuracy of the exhaust gas temperature in a catalytic converter.
Unexamined Japanese patent publication No. 7-180536 discloses a technique for mutually correcting the temperature data obtained from two exhaust gas temperature sensors considering specific conditions.
However, these conventional techniques are insufficient for solving the above-described deficiencies of Japanese patent No. 2593506.
FIG. 11 is a graph showing a relationship between HC flow amount per unit time (g/hr) and upstream/downstream exhaust gas temperature difference (° C.) in relation to catalytic purification rate (η), obtained experimentally through engine tests performed in an emission mode operating region according to which the concentration of combustible substances (HC) flowing into a catalytic converter and the catalytic purification rate (η) are changed variously.
In these tests, measurement of an upstream exhaust gas temperature and a downstream exhaust gas temperature was done accurately. The measuring error of each exhaust gas temperature sensor is negligible.
As understood from FIG. 11, it is difficult to accurately judge the deteriorated condition of a catalyst based on only the upstream/downstream exhaust gas temperature difference.
Furthermore, in the case of diesel engines, a diesel particulate filter (referred to as DPF) is used to trap the exhaust gas emissions called particulates. When a direct-type diesel engine is equipped with a DPF carrying a catalyst oxidizing the combustible substances (HC), it is difficult to directly measure the catalyst temperature in this DPF. It is, hence, necessary to detect the deterioration of the catalyst based on the difference of upstream and downstream exhaust gas temperatures of DPF. Considering such circumstances, it is earnestly requested to provide a high-accurate and non-expensive catalyst deterioration detecting apparatus.